Conventional boost and buck regulators rely on controllable switches to alternately couple and uncouple an inductor between the input and the output of the regulator. Diodes are typically included in the circuit to provide isolation between the common or ground on the one hand, and the input or output of the regulator on the other hand when the switch is closed.
Ideally the switches are either fully ON (with zero voltage drop across the switch) or OFF (zero current), and energy transformation is 100% efficient. In practice this is approximately true. At switching frequencies up to tens of kilohertz the principal losses are the conduction losses in the switch and diode, which are difficult to avoid. There is also a loss during the finite interval spent switching "ON" and "OFF", during which the switch is sustaining both voltage and current. This loss can be minimized with fast switches such as Field Effect Transistors (FETs).
In order to reduce the size of the energy storage components (and thus the physical size of the regulator), conversion frequencies are rising to the 100 KH.sub.s to 1 MH.sub.s range. At these frequencies, two additional power losses become important.
The greater loss is due to "reverse recovery" effects in rectifiers. In practice, P-N junction diodes in the regulator circuit will not cease conducting immediately when the switch turns ON, but will allow reverse current to flow for a short time before "recovering" the ability block reverse current and sustain reverse voltage. This process typically requires 20 to 200 ns in fast rectifiers, depending on construction and voltage rating. Very high dissipative currents can flow while both the switch and the diode are conducting, and the energy loss each switching cycle becomes significant at high switching frequencies. The high momentary "reverse" current in the diode tends to cease fairly quickly, generating troublesome amounts of Electro-magnetic Interference (EMI).
An additional loss at high frequencies is the dissipative discharge of the junction capacity of the switch (and other parasitic circuit capacities) when the switch turns ON. FETs are very fast switches, finding favour at high conversion frequencies. Unfortunately their self-capacity is relatively high, and increases if larger FETs are used to minimize conduction loss.
The reverse current flow in the diode (and EMI upon recovery) can be significantly reduced if the rate at which current drops to zero and reverses is reduced. This could be done by turning the switch ON slowly (or by using a slower switch), but this unfortunately increases switching loss (due to simultaneous current and voltage on across the switch) faster than reverse recovery losses are reduced.
One prior art approach to achieving soft switching is to place an inductor in series with either the main switch, or the diode. While this solution may reduce reversing losses, it does not recover the energy stored in the inductor.
A preferable approach is to use a second, pilot switch and a second inductor to reactively limit the rate of change of current through the diode. This might be done by providing a second inductor connected from the anode of the main diode to the second pilot switch to the common line. An auxiliary diode would be provided between the switch and the regulator output to recover the energy from the second inductor when the pilot switch is OFF. That approach suffers the drawback that the auxiliary diode may not be switched off by the time the main switch switches OFF, resulting in a build up of current in the diode and interfering with proper operation of the circuit.